An integrated circuit, such as a radio frequency (RF) integrated circuit, may include multiple transistor dies that are placed in an integrated circuit package by a die attach machine. A robotic bonding tool may then be used to wire bond the dies to other circuit elements within the package, and to leads of a package leadframe. Such a tool generally includes a surface that detects bond pads or other bond sites of a given die, and determines the height coordinates of these bond pads. The other circuit elements in an RF integrated circuit may include, for example, tuning capacitors.
The two major wire-bonding processes used for electronic package interconnects are wedge bonding and ball bonding. The wedge-bonding process has traditionally been used to form the package interconnects of RF integrated circuits due to its ease in forming the wire bond profiles necessary for optimal RF performance.
Ball bonding provides a more economical and robust process than that of wedge bonding. In ball bonding, a wire is fed through and protrudes from a hole in a bonding tool, commonly referred to as a capillary. An electric arc melts the protruding wire into a ball. The solid ball is pressed and bonded onto the bond site by the bonding head. The ball-contact surface interface is subjected to ultrasonic vibrations, a specified bond force, and heat, for a small amount of time to form a bond between the ball and the bond site. However, the inability of traditional ball bonders to perform ball-bonding operations without harsh terminations at a die surface has created an overwhelming bias against using modern ball-bonding processes for wire bonding RF integrated circuits.
Ball-bumping techniques of modern ball bonders lessen the harshness of wire bond terminations on a primary die and capacitors. A ball bump is a solder ball or stud that is typically placed on a bond site prior to bonding an interconnecting wire. In creating a ball bump, a ball formed in the bonding tool is plastically deformed and bonded to the bond site. Additional recent developments in commercially-available ball bonders include improvements such as the ability to perform two separate reverse motions, ball size reduction, and wire length control. However, the ball size associated with the ball bonder remains very large, requiring the use of larger bond sites. Since optimal RF performance often requires minimal bond site size, the ball sizes are also a factor in favoring the use of wedge bonding instead of ball bonding in the assembly of RF integrated circuits. Methods of ball bonding on previously made ball bumps on the bond sites of a semiconductor device have also been suggested. See, for example, U.S. Pat. Nos. 5,976,964, 6,165,887 and 6,624,059.
A standard ball bond on a smaller bond site may cause contact with a passivation layer and result in a cracked passivation layer and other possible die damage. The use of a thinner wire may decrease the size of the resulting ball bond for bonding purposes, allowing the ball bond to fit on the smaller bond site without contact with the passivation layer. Thinner wire also results in less harsh terminations during bonding. However, when the thinner wire is used to connect circuit elements of the integrated circuit, problems such as wire sweep, which is caused during molding when wires are pushed together, and weakened signal strength may result. Thus, a need remains for further improvements in ball-bonding techniques, particularly in RF integrated circuit applications.